Sintered R-T-B based magnets (where R is a rare-earth element including Y; T is a transition element whose main component is Fe, and is specifically Fe or Fe and Co; and B is boron) are broadly used in various types of electronic equipment because they exhibit the highest magnetic energy product among various magnets and they are relatively inexpensive.
Sintered R-T-B based magnets are produced by the steps described below, for example.
First, a raw material alloy is produced by casting various raw material metals with a method such as an ingot method or a strip casting method. Next, the resultant raw material alloy is subjected to a pulverization step to obtain an alloy powder of a predetermined particle size. This pulverization step usually involves a coarse-pulverization step and a fine-pulverization step, the former being performed by utilizing e.g. the hydrogen embrittlement phenomenon, and the latter being performed by utilizing e.g. an jet mill (airflow pulverizer).
Next, the powder is subjected to a step of pressing in a magnetic field where it is molded into a desired shape while being in a magnetic field. The resultant compact is sintered, whereby a sintered R-T-B based magnet is produced. Moreover, the sintering is usually followed by a heat treatment.
In the step of pressing in a magnetic field, the aforementioned powder is molded while applying a magnetic field thereto in a predetermined direction, whereby the directions of easy magnetization of the respective powder particles are aligned in the direction of the applied magnetic field. Enhancing the degree of alignment (alignment degree) of the axes of easy magnetization of main phase crystal grains of the sintered R-T-B based magnet to be finally obtained allows a sintered R-T-B based magnet with high remanence Br to be obtained.
Patent Document 1 discloses use of a counter jet mill which, in addition to a principal pulverization gas that is introduced through a pulverization gas nozzle, introduces a circulation-generating gas through a separately provided nozzle, whereby fine powder with obtuse ridges can be obtained. This fine powder has good slippage, which promotes the alignment degree of the fine powder when it is pressurized while applying a magnetic field thereto during compacting (or pressing).
Patent Document 2 discloses performing a heat treatment for finely-pulverized rare-earth alloy powder, prior to compacting in a magnetic field. The rare-earth alloy powder having been subjected to the heat treatment acquires a roundish shape due to sphericalization. This reduces the frictional force and steric hindrance during alignment in a magnetic field, thereby improving the alignment degree.